Dimensionally adjustable anode-dimensionally stable diaphragm combination for electrolytic cells

ABSTRACT

A diaphragm cell for the electrolytic production of chlorine and alkali metal hydroxide, in other respects conventional, is improved by the provision of a dimensionally stable, polymermodified diaphragm together with a dimensionally adjustable anode capable of movement toward said diaphragm on cell assembly.

United States Patent [191 OLeary et al.

DIMENSIONALLY ADJUSTABLE ANODE-DIMENSIONALLY STABLE DIAPHRAGMCOMBINATION FOR ELECTROLYTIC CELLS Inventors: Kevin J. OLeary, ClevelandHeights; Charles P. Tomba; Robert W. Fenn, III, both of Painesville, allof Ohio Diamond Shamrock Corporation, Cleveland, Ohio Filed: Mar. 1,1974 Appl. No.: 447,215

Assignee:

US. Cl. 204/282; 204/252; 204/266; 204/283; 204/296 Int. Cl. C25B 11/02;C25B 13/02 Field of Search 204/252, 266, 283, 296, 204/282 ReferencesCited UNITED STATES PATENTS 7/1972 Fogelman 204/252 X Dec. 23, 19753,694,281 9/1972 Leduc 204/296 X 3,803,016 4/1974 Conner, Jr. 204/252 X3,853,721 12/1974 Darlington et a1 204/296 X FOREIGN PATENTS OR,APPLICATIONS 700,296 12/1964 Canada... ..204/296 Primary Examiner-JohnH. Mack Assistant Examiner-W. 1. Solomon Attorney, Agent, or FirmTimothyE. Tinkler [57] ABSTRACT A diaphragm cell for the electrolyticproduction of chlorine and alkali metal hydroxide, in other respectsconventional, is improved by the provision of a dimen- 3 Claims, NoDrawings DIMENSIONALLY ADJUSTABLE ANODE-DIMENSIONALLY STABLE DIAPHRAGM Alarge portion of the worlds production of chlorine and caustic bythe'electrolysis of brine takes place in diaphragm type cells. In suchcells, the adjacent anodes and cathodes are separated by hydraulicallypermeable diaphragms, generally of asbestos fibers.

In the majority of instances, these asbestos diaphragms are formed bydirect deposition of the fibers from a slurry onto the cathode,generally a woven steel mesh. This diaphragm-coated cathode is employedspaced a fixed'distance from the adjacent anode, the latter now being inmost instances a dimensionally stable anode, e.g., titanium sheet orexpanded metal bearing an electrically conductive, electrocatalyticallyactive coating, often based on a platinum group metal.

Owing to the commercial significance of such cells, means by which theperformance thereof may be improved, and especially means by which atleast a por tion of the existing cell hardware may be adapted toimproved-operation, are in great demand.

One proposal for such improved operation is the expandable ordimensionally adjustable electrode of US. Pat. No. 3,674,676. Thiselectrode design allows fabrication and installation of an anode into anelectrolytic cell in a contracted form, thus easing assembly problems.On assembly, the dimensional adjustability comes into play by allowingmovement of the electrolytically active anode toward the diaphragmcoatedcathode without interrupting the electrical integrity of the anodeassembly. This reduction in the interelectrode gap results in a loweroperating cell voltage owing to the reduced path the current must travel.through the resistant brine solution. It has been found, however, thatin many applications insertion and employment of such a dimensionallyadjustable anode in an otherwise conventional diaphragm cell, whileresulting in the desired voltage reduction, also lowers the currentefficiency at which the electrolytic process proceeds. Hence, a means totake advantage of the voltage reduction without loss in currentefficiency is sought. 3

STATEMENT OF THE INVENTION Therefore, it is an object of the presentinvention to provide an improved diaphragm cell for the production ofchlorine and caustic.

It is a further object of the present invention to provide a modifieddiaphragm cell employing many components of existing cells, thusreducing capital outlays.

There has now been found an improvement in a cell for the electrolysisof aqueous alkali metal chloride solutions comprising a containerdivided into anodecontaining and cathode-containing compartments by ahydraulically permeable diaphragm, which improvement comprises, incombination:

a. as said anode, a dimensionally adjustable electrode, and

b. as said diaphragm, a dimensionally stable, thermoplasticpolymer-modified, asbestos diaphragm. Such a cell operates at a lowvoltage and a high current efficiency for extended periods of time andis easy to assemble and service when necessary. The entire cathodeassembly and cover of an existing diaphragm cell may be employed, theonly cell modifications necessary being those required to accommodatethe metal anodes.

DESCRIPTION OF THE PREFERRED EMBODIMENTS Essentially any diaphgram cellfor the electrolysis of alkali metal halide solutions, especially sodiumand potassium chlorides, may be improved by incorporation of thediaphragm and anode combination of this invention. Thus, new designs,such as bipolar cells, may be improved by rendering the anode, thecathode, or both, dimensionally adjustable, that is, by providing formovement of one or both electrodes from the partition separatingadjacent cell units toward the adjacent electrode, the diaphragm beingdisposed therebetween.

However, the invention will find most utility in the modification ofexisting monopolar diaphragm cells, such'as those described in US. Pat.No. 2,987,463. Here the cathode assembly or can and the cell coverremain unchanged. An anode base capable of accommodating and disposingmetal anodes within the cell, such as the metal base that is the subjectof US. Pat.

No. 3,591,483, will be required to replace the oldercopper/lead/concrete/asphalt combination bases used with graphite.

The term dimensionally adjustable anode or electrode refers to thoseelectrodes wherein it is possible on assembly of the cell to move theactive anode surface closer to the diaphragm-coated cathode on assembly.Generally, such electrodes will comprise an active anode sheet or faceconnected to a riser or other primary electrical conductor by a meanscapable of causing the desired movement while maintaining electricalintegrity. The anode face is a metal sheet, continuous or foraminous,generally at least partially coated with an electrically conductive,electrocatalytically active material. The variety of such anodes knownto the art may be employed. Preferably, the anode face will be expandedtitanium metal, to allow for gas release and electrolyte circulation,coated with a platinum group metal oxide or related material. The riseror other conductor serves to convey current from the cell base or asource external the cell to the operating anode face. Conveniently, thismay take the form of a metal pillar, such as copper-cored titanium,extending through the cell base. See, for example, US. Pat. No;3,591,483, incorporated herein by reference. The anode face is thenconnected to this conductor by a moveable, conductive, connecting meanssuch as a flexible titanium sheet. Dimensionally adjustable electrodesof a type useful as the anodes of this invention are described in moredetail in US. Pat. No. 3,674,676, the teachings of which as to electrodeconfiguration, alternatives, and advantages are incorporated herein byreference. Other designs may suggest themselves to one skilled in theart, it only being essential that the requisite capacity to move theactive anode sheet toward the opposed diaphragm-coated cathode on cellassembly be present.

Combined with the dimensionally adjustable anode is a dimensionallystable, polymer-modified, asbestos diaphragm. Dimensional stability withrespect to the diaphragm refers to resistance to swelling in the aqueouselectrolyte as well as resistance to chemical and mechanical attrition.These desirable properties are achieved by treating conventional(usually chrysotile) asbestos fibers with various chemically andmechanically resistant thermoplastic polymers. Included, andparticularly to be preferred, are the various polyfluorocarbons such aspoly(vinyl fluoride), poly(vinylidene fluoride),polytetrafluoroethylene, polyperfluoroethylene propylene, and the like.Also useful are certain of the chlorinated resins such aspoly(vinylidene chloride) and the chloro-fluoro materials such aspolychlorotrifluoroethylene and polychlorotrifluoroethylene polyethylenecopolymers. In addition, it is possible to impart favorable propertiesto the diaphragm if the polymer employed also has cation exchangeproperties. Diaphragms of this type are described in Dutch PublishedApplication 72/ 12225 and exemplified by the use ofpoly(perfluoroethylene-trifluoroethylene sulfonic acid) with asbestos.

These polymers may be incorporated into the asbestos diaphragm bypreforming the asbestos fibers as a diaphragm or sheet and subsequentlysoaking same in a solution of the appropriate monomers, followed by insitu polymerization. In this fashion, however, a continuous polymercoating on the asbestos fibers is obtained and the advantageous ionexchange properties of the asbestos are thereby lost. Generally, thepolymers will be incorporated in the asbestos as such, by eitherdissolving or suspending same in an appropriate media followed bysoaking the preformed asbestos sheet or diaphragm therein or drawing thepolymer into the asbestos, for example, by vacuum. Subsequently, thetreated asbestos structure is dried and cured, i.e., raised to thetemperature at which the thermoplastic polymer melts, thereby lockingtogether adjacent asbestos fibers on cooling and imparting the desireddimensional stability.

A preferred method of obtaining the desired dimensionally'stablediaphragm is by deposition from a slurry of asbestos fibers andthermoplastic polymer particles in the form of polymer granules orfibers. This uniform slurry is then drawn onto the cathode screen orother sheet-forming support, followed by drying and curing at theappropriate elevated temperature to obtain a discontinuous but coherentpolymer coat. This method and the results obtained thereby are describedmore fully in copending US. Ser. No. 324,508, filed Jan. 17, 1973.

In each of the foregoing instances the object is to obtain adimensionally stable diaphragm comprising an asbestos fiber latticestabilized with a thermoplastic polymer that has been subjected tofusion to bind adjacent asbestos fibers together and, preferably,provide a discontinuous polymer coating on said fibers.

Preferably, in order to avoid leakage and obtain the maximum advantage,the dimensionally stable diaphragm will have been applied directly ontothe cathode surface, usually a wire screen. If this is the case, thenthe diaphragm-coated cathode assembly may be positioned over the arrayof dimensionally adjustable anodes to provide the desired alternatinganode-cathode arrangement, following which the anodes may be allowed orcaused to expand, thus reducing the interelectrode gap. The cell maythen be operated conventionally but with the above-described advantages.

We claim:

1. An improvement in a cell for the electrolysis of aqueous alkali metalchloride solutions comprising a container divided into anode-containingand cathodecontaining compartments by a hydraulically permeablediaphragm, which improvement comprises, in combination:

a. as said anode, a dimensionally adjustable electrode comprising ananode-working face, an anode riser, and a moveable means for connectingthe anode working face and anoderiser with electrical integrity, and b.as said diaphragm, a dimensionally stable, heat-fused,polyfluorocarbon-asbesto's diaphragm, said diaphragm having adiscontinuous polyfluorocarbon coating binding the asbestos together.

2. A cell as in claim 1 wherein said polyfluorocarbon ispolytetrafluoroethylene.

3. An improvement in a cell for the electrolysis of aqueous alkali metalchloride solutions comprising a container divided into anode-containingand cathodecontaining compartments by a hydraulically permeablediaphragm, which improvement comprises, in combination:

a. as said anode, a dimensionally adjustable electrode comprising ananode-working face, an anode riser, and a moveable means for connectingthe anode working face and anode riser with electrical integrity, and

b. as said diaphragm, a dimensionally stable, heatfused,polymer-asbestos diaphragm, said polymer being selected from the groupconsisting of poly(- vinylfluoride), poly(vinylidene fluoride),polytetrafluoroethylene, polyperfluoroethylene propylene, andpolychlorotrifluoroethylene-polyethylene, said diaphragm having adiscontinuous polymer coating binding the asbestos together.

1. AN IMPROVEMENT IN A CELL FOR THE ELECTROLYSIS OF AQUEOUS ALKALI METALCHOLRIDE SOLUTIONS COMPRISING A CONTAINER DIVIDED INTO ANODE-CONTAININGAND CATHODE-CONTAINING COMPARTMENTS BY A HYDRAULICALLY PERMEABLEDIAPHRAGM, WHICH IMPROVEMENT COMPRISES, IN COMBINATION: A. AS SAIDANODE, A DIMENSIONALLY ADJUSTABLE ELECTRODE COMPRISING AN ANODE-WORKINGFACE, AN ANODE RISER, AND A MOVABLE MEANS FOR CONNECTING THE ANODEWORKING FACE AND ANODE RISER WITH ELECTRICAL INTEGRITY, AND B. AS SAIDDIAPHRAGM, A DIMENSIONALLY STABLE, HEAT-FUSED, POLYFLUOROCARBON-ASBESTOSDIAPHRAGM, SAID DIAPHRAGM HAVING A DISCONTINUOUS POLYFLUOROCARBONCOATING BINDING THE ASBESTOS TOGETHER.
 2. A cell as in claim 1 whereinsaid polyfluorocarbon is polytetrafluoroethylene.
 3. An improvement in acell for the electrolysis of aqueous alkali metal chloride solutionscomprising a container divided into anode-containing andcathode-containing compartments by a hydraulically permeable diaphragm,which improvement comprises, in combination: a. as said anode, adimensionally adjustable electrode comprising an anode-working face, ananode riser, and a moveable means for connecting the anode working faceand anode riser with electrical integrity, and b. as said diaphragm, adimensionally stable, heat-fused, polymer-asbestos diaphragm, saidpolymer being selected from the group consisting of poly(vinylfluoride),poly(vinylidene fluoride), polytetrafluoroethylene,polyperfluoroethylene propylene, andpolychlorotrifluoroethylene-polyethylene, said diaphragm having adiscontinuous polymer coating binding the asbestos together.